[time-nuts] Discipline a rubidium driven net4501 with a Garmin LVC 18?

[Jason Rabel]

Nothing wrong with wanting to use a Rb as the clock source for the board. I *almost* decided to go that route since I had a spare
LPRO sitting around. But I realized that I would never need that long of hold-over so it would kind of be a waste. I've been looking
at some cheap ($20 USD) 1PPM TCXO on ebay... Which would be a decent upgrade from the stock crystal rated at 50PPM.

> Furthermore I'd like to get a 1PPS input from GPS as others already did. 
> However, I would like to use one of these Garmin GPS 18 LVC units (as on 
> http://time.qnan.org/) that usually work fine and provide a 1PPS signal 
> together with a NMEA output and connect it to the net4501 internal 
> serial port instead of the FatPPS (as John did: 
> http://www.febo.com/pages/soekris/). Is there a way to get 
> Poul-Hennings`s NTPns working with the GPS 18 as a PPS source?

I'm not 100% sure, but I *think* there is a basic PPS driver with NTPns? On my box I set the course time on boot-up via ntpdate,
then when NTPns starts it has something decent to work with (if the time is too far off I think it throws an alarm).

You can get Motorola Oncore UT+ or even M12+T receivers on eBay relatively cheap. With a little effort they *will* fix inside the
soekris box so you can just have a connector on the back for your antenna. The UT+ receivers *will* work with NTPns.

You can checkout my first build here:
http://www.extremeoverclocking.com/articles/howto/Building_S1_NTP_Server_1.html

> Is there any specific reason why a 1PPS signal from another source (like 
> a Thunderbolt GPS disciplined clock) together with a FatPPS board would 
> give better results as my Garmin approach?

I *think* the PPS output from a Thunderbolt is not the raw GPS PPS but rather a deterministic one? Someone will have to verify /
debunk that though. However you are also kind of doubling up on the oscillators since when a Thunderbolt looses signal it will
flywheel off it's internal OCXO.

I have a pre-built image of NanoBSD w/NTPns (and I think regular NTP is on there too) that I did a while back, feel free to give it
a whirl:

http://www.rabel.org/ntpns/ntpns_NanoBSD_7.tar.bz2

You might have to change some of the startup paramaters & ntpns configurations depending on how you wire up everything to the GPIO
pins and whatnot.